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Characteristics and Origin of Earth-Mounds on the Eastern Snake River Plain, Idaho

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Characteristics and Origin of Earth-Mounds on the Eastern Snake River Plain, Idaho INEL-95/0505 Characteristics and Origin of Earth-Mounds on the Eastern Snake River Plain, Idaho J. A. Tullis Published September 1995 Idaho National Engineering Laboratory Lockheed Martin Idaho Technologies Idaho Falls, Idaho 83415 Prepared for the U.S. Department of Energy Assistant Secretary for Environmental Management Under DOE Idaho Operations Office Contract DE-AC07-94ID13223 MASTER DISTRIBUTION OF THIS DOCUMENT IS UNUMflEEi ABSTRACT Earth-mounds are common features on the Eastern Snake River Plain, Idaho. The mounds are typically round or oval in plan view, <0.5 m in height, and from 8 to 14 m in diameter. They are found on flat and sloped surfaces, and appear less frequently in lowland areas. The mounds have formed on deposits of multiple sedimentary environments. Those studied included alluvial gravel terraces along the Big Lost River (late Pleistocene/early Holocene age), alluvial fan segments on the flanks of the Lost River Range (Bull Lake and Pinedale age equivalents), and loess/slopewash sediments overlying basalt flows. Backhoe trenches were dug to allow characterization of stratigraphy and soil development. Each mound has features unique to the depositional and pedogenic history of the site; however, there are common elements to all mounds that are linked to the history of mound formation. Each mound has a "floor" of a sediment or basement rock of significantly different hydraulic conductivity than the overlying sediment. These paleosurfaces are overlain by finer-grained sediments, typically loess or flood-overbank deposits. Mounds formed in environments where a sufficient thickness of fine-grained sediment held pore water in a system open to the migration to a freezing front Heaving of the sediment occurred by the growth of ice lenses. Mound formation occurred at the end of the Late Pleistocene or early in the Holocene, and was followed by pedogenesis. Soils in the mounds were subsequently altered by bioturbation, buried by eolian deposition, and eroded by slopewash runoff. These secondary processes played a significant role in maintaining or increasing the mound/ intermound relief. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees makes any warranty, express or implied, or assumes any legal liability or responsi• bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights Refer• ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom• mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. iii DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. CONTENTS ABSTRACT iii INTRODUCTION 1 BACKGROUND 2 Objective 2 Theories of Mound Origin 2 Mammal Activity 2 Erosional Hypotheses 3 Depositional Hypotheses 3 Seismic Shaking 3 Frost Hypotheses 4 Previous Investigations 5 Regional Setting 5 STUDY APPROACH 13 Field Methods 14 Laboratory Analyses 14 Sample Preparation 15 Saturated Paste 15 pH 15 Electrical Conductivity 15 Calcium Carbonate Equivalent 15 Pipette Analysis 16 SITE AND TRENCH DESCRIPTIONS 18 Alluvial Fan Mounds 18 Introduction 18 8MFM2 23 8MFM2X 24 8MFM4 29 Fluvial terraces 35 Introduction 35 v BLRM2 36 BLRM4 45 Loess-Over-Basalt Mounds 46 Introduction 46 RWMCM1 46 IPM2 56 RGFM1 65 DISCUSSION 81 Regional Paleoclimate 81 Mound Physiography 81 Vegetation 81 Mound Floors 82 Stratigraphy 82 Pedogenesis 83 Mammal Activity 84 Secondary Processes 84 CONCLUSIONS '. 85 REFERENCES 86 Appendix A—Partial List of Mound Origin Research Since 1945 A-l Appendix B—Field and Laboratory Data B-l Appendix C—Site Location and Surficial Characteristics C-l FIGURES 1. Computer-generated, digital-topographic map showing the regional setting, and the contrast in relief between the ESRP, and the Basin and Range Mountains and Yellowstone Plateau 7 2. Generalized geologic map of the INEL area showing the distribution of basalt flows and thick sedimentary deposits 9 vi Soil textural classes plotted on a triangular diagram 17 USGS 7.5 minute topographic map showing the location of 8MFM2 and 8MFM4 in relation to the Eight Mile Fan 19 Profile log of 8MFM2 in envelope Profile log of 8MFM4 in envelope USGS 7.5 minute topographic map showing the location of BLRM2 and BLRM4 41 Profile log of BLRM2 in envelope Profile log of BLRM4 in envelope USGS 7.5 minute topographic map showing the location of RWMCMl and IPM2 51 Profile log of RWMCMl in envelope Profile log of IPM2 in envelope USGS 7.5 minute topographic map showing the location of RGFM1 67 Profile log of RGFM1 in envelope TABLES Late Pleistocene-Holocene climate and chronology of the ESRP and vicinity 12 Summary of trench environment, location, and names 13 PLATES Aerial photograph of Eight Mile Fan at a scale of 1:40,000 21 Intermound soil on the left side of 8MFM2 25 Central disturbed section of 8MFM2 25 Soil crack in 8MFM2 27 Intermound soil position at left end of trench 8MFM4 31 Disturbed soil in the central section of 8MFM4 31 vii 7. Soil on the right side of trench 8MFM4, showing the basal gravels 33 8. Aerial photograph of the Big Lost River showing the Qa and Qb terraces and the locations of trenches BLRM2 and BLRM4 37 9. Photograph showing the difference in pavement between the mound and intermound surfaces 39 10. Intermound soils at the left side of BLRM2 43 11. Disturbed soils in the central section of BLRM2 43 12. Intermound soil at right side of BLRM4 47 13. Intermound soil at left side of BLRM4 47 14. Central disturbed section in BLRM4 49 15. Aerial photograph showing the location of RWMCM1 and IPM2 53 16. Photo showing the buried rock lines and soils in RWMCM1 57 17. Soil profile near the left-center section of RWMCM1 57 18. Photograph of the disturbed soil near the center of RWMCM1 59 19. Photograph showing the entire length of IPM2 61 20. Intermound soil at right end of IPM2 61 21. Central disturbed zone in IPM2 63 22. Frost heaved basalts in the base of IPM2 63 23. Air photo showing the location of RGFM1 69 24. Photograph showing the surface of RGFM1 71 25. Photograph showing the downslope intermound soil at the left of RGFM1 71 26. Photograph showing the upslope intermound soil in RGFM1 73 27. Photograph of a wedge in the intermound soil of RGFM1 remnant soils 77 28. Photograph showing soil fractures in the intermound soil of RGFM1 77 29. Photograph showing the central disturbed zone in RGFM1 19 viii V Characteristics and Origin of Earth-Mounds on the Eastern Snake River Plain, Idaho INTRODUCTION This report was originally published in May 1995 as a Master of Science thesis from Idaho State University. The work was partially funded by the Department of Energy, and was published here to allow greater distribution of the results. Earth mounds on the Eastern Snake River Plain (ESRP) are subtle landforms often overlooked by casual observation. They are generally round or oblong, with diameters ranging from 8-14 meters. Their highest point is typically less than a half-meter above the adjacent intermound area. Sloping sides and gradational margins make the mound/intermound boundaries difficult to pinpoint. However, vegetational differences between the mounds and intermounds are striking, simplifying their identification. Vegetational differences vary; e.g., areas dominated by sagebrush communities usually have more and taller sagebrush on the mound; and areas dominated by grass communities usually have increased abundance of annual species on the mound. Adjacent mounds are always separated by some distance, usually up to several times the diameter of the mounds. The observed distribution patterns vary from an apparent regular spacing, to random occurrences, to strongly linear alignment They formed on a variety of sedimentary deposits including river terraces, alluvial fan surfaces, and loess deposits on basalt flows. The mounds occur on both flat and sloping surfaces. Essentially all mounds observed in this study are, or have been, occupied by burrowing mammals. Similar earth mounds are found elsewhere in the world, but in the United States they generally occur west of the Mississippi River. Origins of the North American mounds puzzled early explorers and settlers, and have continued to puzzle scientists since the 19th century. North American mounds were labeled with many names including Mima mounds, mimalike mounds, patterned ground, pimple mounds, hogwallows, silt mounds, and pimple prairie. Some names are associated with specific locations, i.e., Mima mounds for mounds on the Mima Prairie in Thurston County, Washington; and patterned ground for soil features formed in periglacial environments, or areas of permafrost. Although the ESRP mounds share many characteristics of mounds in other areas, they are called "mounds" here for simplicity and not to imply any genetic connections. The origin and age of mounds found in different geographic regions are puzzling because the environments they formed in exhibit such a wide range of variability. Their origins have been studied by researchers in the fields of geomorphology, geocryology, pedology, biology, and ecology (among others). It is possible that processes from any or all of those fields may relate to their formation. Although the morphology of mounds can be superficially similar from region to region, there is no universally accepted theory for their origin. 1 BACKGROUND Objective The objective of this research was to determine the age and origin of earth mounds on a segment of the ESRP within the Idaho National Engineering Laboratory (INEL).
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